Recently, there is an increasing demand for small and power-saving actuators as small communication devices such as cell-phones have widely become in use. In view of such circumstances, a variety of piezoelectric actuators using a piezoelectric element as a vibration drive source and realizing a small size and power saving have been developed. A piezoelectric element is an element having piezoelectric effect and inverse piezoelectric effect. A piezoelectric element generates a voltage when an external force such as vibration or a pressure is applied thereto and, conversely, vibrates (displaces) when a voltage is applied thereto.
A piezoelectric actuator generates mechanical vibration as a piezoelectric element within a piezoelectric vibrator vibrates. For example, as an electronic part of cell-phones, piezoelectric actuators have applications in vibration devices such as vibrators and acoustic devices such as speakers.
The piezoelectric vibrator of the above piezoelectric actuator includes those of the bimorph type having piezoelectric elements joined on either side of a base and those of the unimorph type having a piezoelectric element joined on one side of a base.
Generally, the bimorph piezoelectric vibrator has advantages such as a high vibration drive power and large vibration amplitudes compared with the unimorph piezoelectric vibrator.
FIG. 13 is an exploded perspective view of a piezoelectric actuator 101 having a bimorph piezoelectric vibrator. As shown in the figure, the piezoelectric actuator 101 has a pair of upper and lower piezoelectric elements 11 having a piezoelectric ceramic plate as an piezoelectric body, a base 12 to which the pair of upper and lower piezoelectric elements 11 are joined, an annular vibrating membrane 13 of which the inner peripheral part of the top surface is joined to the outer peripheral part of the undersurface of the base 12, and an annular support member (frame) 15 connecting and supporting the vibrating membrane 13. The pair of upper and lower piezoelectric elements 11 and base 12 constitute a piezoelectric vibrator (vibration generator) 10. A pair of electrode layers (not shown) as a conductor is formed on the top surface 11a and undersurface 11b of each piezoelectric element 11.
If the piezoelectric actuator 101 has a diameter (the length on a side) of 20 mm so that it can be used as an electronic part of a cell-phone, the piezoelectric actuator 101 has a fundamental resonance frequency f0 of 2 kHz or higher.
FIG. 14(A) is a cross-sectional view of the core part of the piezoelectric actuator 101 having the above-described bimorph piezoelectric vibrator and FIG. 14(B) is a schematic illustration showing the vibration of the piezoelectric actuator 101.
As shown in FIGS. 14(A) and 14(B), as an alternating-current voltage is applied to the pair of upper and lower electrode layers of the piezoelectric element 11 to generate an alternating-current electric field within it, the piezoelectric element 11 radially expands/contracts. Since the piezoelectric element 11 is joined to the base 12 so as to restrain its expanding/contracting motions, as shown in FIG. 14(B), the base 12 flexes in the direction perpendicular to the joint surface to the piezoelectric element 11 as the piezoelectric element 11 expands/contracts. In other words, the base 12 repeatedly deforms into the convex mode indicated by the solid lines and into the concave mode indicated by the broken lines in FIG. 14(B). Then, as the base 12 deforms in such a manner, the inner peripheral part of the vibrating membrane 13 vibrates in the up-and-down (vertical or perpendicular) direction. Then, as the inner peripheral part of the vibrating membrane 13 vibrates, the base 12 and vibrating membrane 13 flex (vibrate) in the direction perpendicular to the top surface and undersurface (which are also termed the main surfaces) of the base 12, during which the connection part to the support member 15 serves as the fixed end and the center part of the base 12 serves as the largest amplitude part.
Generally made of a highly rigid material such as ceramics (a piezoelectric ceramic plate), the piezoelectric element 11 tends to vibrate with small vibration amplitudes. Therefore, the above-described piezoelectric actuator 101 tends to have a low level of vibration amplitude and/or sound pressure compared with an electrodynamic actuator using an electromagnetic force acting between a permanent magnet and a coil as the vibration drive source. Here, the term “rigidity” refers to a property of an object that is determined by the Young's modulus E (Pa) and form factors such as thickness and shape.
FIGS. 15 and 16 show a piezoelectric actuator 102 of another modified embodiment that is different from the above-described modified embodiment, namely the piezoelectric actuator 101. The piezoelectric actuator 102 is of the bimorph type and has the same structure as the piezoelectric actuator 101 shown in FIGS. 13, 14(A), and 14(B) except that the vibrating membrane 13 is clamped by a pair of upper and lower support members 15m and 15n. 
When an actuator such as a piezoelectric actuator and electrodynamic actuator is used as a vibration device, it is desirable that the actuator vibrates with large vibration amplitudes in a specific frequency band. On the other hand, when an actuator is used as an acoustic device, it is important to take into account the frequency property of the vibration amplitude of the actuator. In other words, an acoustic device requires not only a high level of sound pressure (vibration amplitude) but also an even (flat) frequency property (for example, 80 dB±10 dB) in a frequency band of 500 Hz to 10 kHz, which is the important audible range in terms of auditory sense in order to faithfully reproduce the sound from acoustic wave signals contained in electric signals supplied to the actuator.
On the other hand, Patent Literature 1 and 2 disclose techniques for augmenting the vibration amplitude of a piezoelectric actuator using a bimorph piezoelectric vibrator.
Patent Literature 1 discloses an embodiment in which the piezoelectric actuator is applied to a vibration device. FIG. 29 of Patent Literature 1 discloses a piezoelectric actuator in which a base to either side of which a piezoelectric element is joined is supported by a support member via a vibrating membrane. The vibration of the piezoelectric elements is restricted by the base and converted to flexing motion. Multiple beams are provided at the outer peripheral part of the base. The vibration of the base is transmitted to the beams and further transmitted from the beams to the vibrating membrane so that a larger magnitude of vibration is obtained.
On the other hand, Patent Literature 2 discloses an embodiment in which the piezoelectric actuator is applied to an acoustic device. This piezoelectric actuator comprises a casing, an annular support member curved between the inner and outer peripheral parts and of which the outer peripheral part is fixed to the inner surface of the casing, and a piezoelectric vibrator fixed to the inner peripheral part of the support member and constructed by joining a piezoelectric element and a vibrating plate. With the vibrating plate and casing being coupled via the curved support member, the piezoelectric vibrator vibrates both in the direction perpendicular to the plate surface of the vibrating plate and in the direction parallel to the plate surface of the vibrating plate.